Refractive index measurement by fiber point diffraction longitudinal shearing interferometry

Experimental ray-tracing with point diffraction interferometry and its application in focal length measurement

A novel method that could experimentally trace the ray propagation of an optical system based on point diffraction interferometry (PDI) is presented. The ray represented by the line connecting two point light sources (PLS) intersects with two parallel photographic planes, which are separated at a distance. The intersections of the ray defined by the PLSs with the planes occur where the optical path difference is a maximum and can be determined from interferograms on the photographic planes. The ray is determined by connecting the two intersections. Using fibers as the PLSs and CCD arrays as the photographic planes, we demonstrate its principle and its application in focal length measurement through experiments.

Reflective long-period fiber grating-based sensor with Sagnac fiber loop mirror for simultaneous measurement of refractive index and temperature

In this paper, we propose a reflective long-period grating-based sensor with a Sagnac fiber loop mirror (SFLM) for simultaneous measurement of refractive index (RI) and temperature. By cascading the SFLM to the end of a long-period fiber grating (LPFG), the LPFG works as a reflection operation, which is convenient in some applications. Further, the SFLM and the LPFG have different sensitivities to RI and temperature. As a result, RI and temperature measurement can be simultaneously achieved by monitoring the wavelength shifts of the LPFG and the SFLM's dips in the reflection spectrum. Experimental results show that the temperature sensitivity can reach 1.533 nm/°C, and the RI sensitivity is from 16.864 nm/RIU (refractive index unit) to 113.142 nm/RIU when the RI range is from 1.333 to 1.430. The application for 40 km long-distance RI and temperature measurement shows that the sensor has potential application in long-distance sensing.

Refractive index measurements of multiple layers using numerical refocusing in FF-OCT

We propose and demonstrate the novel method of refractive index (RI) measurement for each layer of multilayered samples, which is based on numerical refocusing in full field optical coherence tomography (FF-OCT). The en-face FF-OCT image on an inner layer boundary of a multilayered sample is unintentionally blurred or defocused due to the RI of the sample itself, but can be numerically refocused. The refocusing is performed by numerically shifting the image sensor plane of the system, in general. However, by calculating the corresponding sample shift and then compared it with the actual sample shifting distance, we could extract the average RI of the layer between any two layer boundaries within the multilayered sample. In addition, the thickness of that particular layer could be derived at the same time. For the idea proof, several samples were prepared by stacking, for each sample, two transparent plates with a gap in between. While changing the material of the plate and filling the gap with oil, the RIs of the plate and the oil were measured. For oils of various RIs, from 1.2977 to 1.3857, the measured RIs were well matched with the reported ones within 0.205%. Moreover, even with a stack of various and multiple plates in front of the same oil layer, the oil RI and the physical thickness of the oil layer were extracted with average errors of only 0.065% and 0.990%, respectively.

Lens refractive index measurement based on fiber point-diffraction longitudinal interferometry

A non-immersive lens refractive index measurement method based on fiber point-diffraction longitudinal interferometry is presented. The lens imaging process is simplified to the single refraction of the back surface if the object point is located at the vertex of the front surface. The lens refractive index is derived through measuring its thickness, radius of curvature of the back surface, the distance between the object point and the image point. Experiments indicate its accuracy is better than 2.2 × 10⁻⁴. Since the front surface is excluded in the imaging process, even an aspherical lens could be accurately measured by this method.